1. Field of the Invention
The present invention relates to a method and an apparatus for vacuum treatment such as decarburization degassing, removal of inclusions and bath temperature adjustment of various molten steels, more efficiently and economically than conventional arts using the R-H (Ruhrstahl-Heraeus) process.
Along with the increasing demands for higher grades of speciality steels, a more efficient and economical production process has been required. To meet with this requirement the refining of molten steels under vacuum has been an indispensable treatment method.
2. Description of Prior Arts
However, the convention vacuum degassing processes, such as the R-H process and the D-H (Dortmund-Hoerder) process, have the following problems to be still solved, which have been hindrances in their commercial operations.
I. The convention processes aim only at the degassing of molten steels and do not provide decarburization of the molten steels so that they can not prevent increase of carbon content in the molten steel and require use precious low-carbon or super-low carbon ferro-alloys;
II. The conventional processes have no ability to increase the bath temperature during the treatment so that the lowering of the bath temperature during the treatment must be compensated for by increasing the blow-off temperature of a converter. This causes unavoidable wearing of the converter refractories.
In order to solve the above difficulties, it has been proposed to oxidize the molten steel bath under a reduced pressure and it has been known to blow pure oxygen onto a molten steel flowing in a vacuum refining vessel of R-H degassing apparatus from a tuyere provided above the molten steel bath. This method has solved the above problem (i) only and has been confronted with by the following unsolved problems.
1. As the gas is blown to the molten steel bath from a position above the bath surface, the molten steel splashes a great deal due to the shock caused by the blown gas.
2. There is insufficient contact between the bath and the gas so that efficiency of O.sub.2 (for example, oxygen efficiency in respect to decarburization) is low.
3. As the gas is blown from above the bath, the slag, etc., if any, floating on the bath surface, hinders the contact between the gas and the molten steel, so that the efficiency of the O.sub.2 is lowered.
4. The oxidation loss of alloying elements can not be eliminated completely. For example, in vacuum degassing of stainless steels, the chromium content is lost by 0.2 to 0.6% in any of the conventional processes, thus causing a production cost increase.
5. The finishing temperature can not be controlled precisely due to the oxidation loss of the alloying elements.
6. As the oxygen efficiency for decarburization varies widely in a low range from 40 to 70%, the final carbon content can not be controlled satisfactorily.